Darrell Markewitz is a professional blacksmith who specializes in the Viking Age. He designed the living History program for L'Anse aux Meadows NHSC (Parks Canada) and worked on a number of major international exhibits. A recent passion is experimental iron smelting.
'Hammered Out Bits' focuses primarily on IRON and the VIKING AGE

Friday, June 27, 2014

Have
you ever used a slag-pit type furnace? I was trying to wrap my head
around one of those the other day, when I read an article saying the pit
was filled with tamped dry straw which allowed the slag to settle into
the pit slowly, leaving the bloom free to grow and extract.

One
of the problem those of us looking at historical bloomery furnaces run
into is that there is no clear typography established to describe
various furnace designs. As the primary archaeological remains are
almost always at best a slag bowl in place inside the vary base of some
enclosing walls, much of what is described is based only on slag. Bad
news for us attempting to actually * make * iron, knowing about only the
base construction hardly gives you enough to build a correctly *
working * furnace.
Most of the current technical knowledge of
effective bloomery iron furnaces comes from the pioneering work of Lee
Sauder & Skip Williams, along with an additional decade of trial and
errors by the 'Early Iron Group' (via Lee's 'Smeltfest' workshops).

(Feel
free to discount the following, but this is more or less how I see
it. Note that this is for European historic designs only. I have never
worked with a Japanese Tatara system, and only once with one of the
African based furnaces)

The basic European model is a Slag Taping furnace. These furnaces are set on ground level.
This may be set up to allow for :'Incontinence' (self taping)Tap Arch (periodic larger taps via an opening in the base)Continuous Taping (ongoing via a slot cut upwards from the base)

In Denmark, many of the bloomery furnaces I saw in use were what they described as Slag Room
furnaces. The furnace is set at ground level, but with the tuyere
elevated to leave extra room for a chamber for slag to collect into.

Ancient Danish and also some Anglo Saxon furnaces are full Slag Pit
furnaces. There the furnace is at ground level, but underneath is dug a
pit of roughly furnace diameter for the slag to drain into.
This
pit is filled with some kind of vegetation - * the pieces set on end
like a bundle of drinking straws * (may be a sheaf of grass or small
twigs). Initially un-burned charcoal (below tuyere level) rests on the
tops of the supporting vegetation. As the slag forms, it can drip down
between the individual pieces. Eventually the heat chars and burns away
the tops of the supports, and so the whole slag bowl can sag downwards
as it enlarges.

This
is an image of an 'Iron Age' (c BC/AD) slag mass from a Danish Slag Pit
furnace, the mass is about the size of a bushel basket, and is upside
down. (from a location near the base of the Jutland, Denmark)

Thursday, June 19, 2014

This will be a fast overview, mainly as a photo essay, on the bloomery iron smelt carried out at Wareham on June 14, 2014.

The prototype is Culduthel, Scotland, circa 200 - 400 AD.
The furnace is a 'slag room' type, with a clay shaft built on a withy frame, over a stone base. The general details of the layout were similar to other Late Iron Age / Viking Age furnaces built in the past. The material used was a 50 / 50 mix of dry clay and course sand.
(See the earlier post : Scottish Dark Ages Iron Smelt )

This smelt was a test / training for the 'Turf to Tools' project being undertaken at the Scottish Sculpture Workshop over August 9 - 25, 2014.

Clay shaft being built up on a wicker frame, over a stone base chamber.

After drying fire - extensive cracking of the shaft (extraction point to right)

Equipment set up, at pre-heat phase.

Adding the DD-SSW1 analog

Bloom mass at initial extraction. Bottom pull was intended, but top extraction was required.

After two initial compaction heats, an attempt to cut the bloom.

Final Bloom : 5.2 kg (from 28.3 kg dry ore = 18% yield)

The next day, the bloom was re-heated and compacted using the hydraulic press. After cutting to quarters, the interior surface was spark tested. The observation suggests roughly .6 -.7 carbon content, a good blade making material. (!)

The following will be of more interest to the archaeologicallly inclined:

The overall remains from the smelt around the furnace.

Broken pieces of the furnace, clearly showing the imprint of the wicker interior frame.

Furnace opened along the major cracks, showing the slag bowl in place below the tuyere entrance.

Main slag bowl removed as two pieces. The tuyere was to the left.

Cleaned surface of the rock base. Tuyere was to the top.

There will be a fuller report to come on making a comparison between the results here, and the remains seen in the archaeology from Culduthel.

Some General Conclusions:

Some modifications to the layout and construction of the furnace may be necessary to :

avoid cracking

conform more closely to the slag pattern at Culduthel

The analog used proved quite successful, even if the yield was a bit low.

The general progress of the smelt followed the expected pattern.

A better system needs to be established for re-heating the bloom for consollodation.

Tuesday, June 17, 2014

This is a great video available on YouTube, featuring long time experimenter, skilled blacksmith and artist Thijs van de Manakker.

The film runs roughly 25 minutes (!) with the historic framework set at c 900 AD.

Although the explanations are a bit simplistic, the activities shown are complete. Paying attention to the entire sequence will give you a lot of insight on furnace construction especially.

Thijs pioneered the use of a twin tuyere system, using two drum bellows. The filmed sequences can give a good estimation of air volumes. One nice addition is the second step, showing the specialized forge set up he uses for the bloom to bar phase.

As a fellow iron maker, I would have liked to have heard some details on ore quantities, addition rates, and yields. But honestly, this kind of technical information is not the intention of the filming

There are a lot of background sequences shot at the Bergherbos Montferland Medieval Heritage site in the Netherlands. The site - and the people there - look just wonderful. (Only a few small inconsistencies caught my eye.)
One thing that did concern me was the complete lack of any safety glasses being used. (Although is is certainly historically accurate, it is just plain not very smart around an iron furnace!)

(I was having trouble accessing the direct 'plug and play' link off YouTube. Hopefully one of these two links will take you direct to the full video...)

The film shows some excellent re-creation work by Thijs. It will certainly be of interest to those involved in living history for the Early Medieval / Viking Age period. Most especially worth the viewing for any considering / working towards historic type bloomery iron smelting furnaces.

Turf to Tools is a two week long collaboration project, running from August 9 - 25. The underlaying intent is to use locally available materials to smelt iron using historic methods, then take the iron created to produce a replica of a specific object - also based on a specific local historic source.

The target object is an axe, the one seen on the 'Rhynie Man' picture stone:

The stone is from nearby Rhynie, and is dated to roughly 700 AD (what was a major political centre at that time). The date makes it clearly 'Pictish' (post Roman and pre Norse).

What is likely the best reference for the iron smelting process is from the archaeological site of Culduthel, which is located at the SW corner of Inverness (about 125 km roughly NW). This site is a longer term occupation, but the main iron smelting activities appear to bracket 200 - 400 AD. There were multiple bloomery furnaces uncovered there.

I have not been able to get a lot of details on the furnace archaeology, but some hints from the partial report and summary I do have suggest:

As the size and general layout (and honestly, historic period) does match my main experimental practice, most of the working system prototype is like that earlier work:

For this prototype smelt :

1) Use of 'standard' ceramic tuyere
2) Electric blower for air

I was able to scrounge a wicker laundry basket to use for the interior form on the build. This is tapered, which is an advantage. This form is 60 cm tall, 25 at the top diameter, 38 at the bottom.
Since there is no specific reference made to cobb or organics added to the upper clay construction at Culduthel, the mix will be roughly 50 / 50 clay and sand.

The charcoal I have on hand is Oak. Currently I don't know exactly what species will be available at SSW, but (more or less) this is not considered a major concern at the test phase.

One of the larger unknowns is ore.
I had made an earlier general commentary on the local 'Macaulyite' ore :

This material is specific to the Lumsden / Rhynie area. (Unfortunately, the Culduthel report does not go into any specifics about any ore that might have been recovered at the site.)

One of the DARC team (our in house geologist Marcus), has suggested the following for the active chemistry of Macaulyite:

Fe2O3 = 78 %

SiO2 = 10.5 %

Al2O3 = 4 %

H2O = 7 %

With a weight of Fe of 60%, this should be a nicely yielding ore. The combined silica and alumina are a bit high however. I was sent a small sample of the available Macaulyite, and the particle size is about the same as rice grains - which is a bit on the small size.

I have made up a version of our dependable 'DARC Dirt' analog, with potter's oxide as the primary component and additions to match at least roughly the chemistry above.

'Spanish Red' oxide = 25 kg

Alumina = .5 kg

Silica = 1.25 kg

Wheat Flour = 2.5 kg

This is dry weight, there is always considerable water weight remaining (which we will calculate via a small sample later).

Expect several more postings detailing furnace build, iron smelt and the bloom to bar phase!

Note : The smelt itself will take place at Wareham on Saturday June 14, starting about 9 AM, with extraction expected some time between 4 - 6 PM. As with past experimental smelts, this is a 'semi open' event : The truly interested can attend, but please (!) get in contact with me via e-mail if you want to come up. Do expect my major focus to be on this prototype smelt, a training and rehearsal for the SSW project.

Thursday, June 05, 2014

The
Dark Ages Re-Creation Company will be mounting a presentation of daily
life / life of the artisan during the Viking Age as part of the larger
Medieval Festival at Upper Canada Village this weekend.

The
core of the DARC living history presentation will be two complete
'camp' set ups, illustrating aspects of daily life, including domestic
tasks centred on food preparation. Additionally, there will be four
primary working artisan demonstrations ongoing :

Subscribe

Ontario Arts Council

Canada Council

Ontario Arts Council

February - May 2012 : 'Bloom to Bar' Project Grant

February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

COPYRIGHT NOTICE -
All posted text and images @ Darrell Markewitz. No duplication, in whole
or in part, is permitted without the author's expressed written permission.
For a detailed copyright statement : go HERE